The present invention relates to a high voltage generating apparatus for supplying a high voltage to an anode of a cathode-ray tube, particularly a high voltage generating apparatus provided with a circuit for stabilizing a high voltage output.
FIG. 1 shows a conventional high voltage generating circuit. This high voltage generating circuit is provided with a horizontal deflection output circuit 1 and a flyback transformer 2.
The horizontal deflection output circuit 1 comprises a horizontal output transistor 4, a damper diode, an oscillation capacitor 6, a horizontal deflection coil 7 and an S-shaped compensation capacitor. The horizontal output transistor 4 receives a voltage pulse transmitted from the horizontal drive circuit to perform switching and applies a sawtooth wave current to the horizontal deflection coil 7 in cooperation with the damper diode 5. On the other hand, the oscillation capacitor and the horizontal deflection coil 7 generates a flyback pulse with their oscillating action and supply it to the flyback transformer.
The flyback transformer 2 is made up with a low voltage coil 11 and a high voltage coil 12 mounted on the core 10 and one end of the low voltage coil 11 is connected to the collector of the horizontal output transistor 4 and the other end of said coil 11 is connected to the power supply 13. The high voltage side of the high voltage coil 12 is connected to the anode of the cathode-ray tube through the high voltage rectifying diode 14 and the other end of said coil 12 is connected to the ABL (Automatic Brightness Limiter). This flyback transformer 2 boosts the flyback pulse voltage supplied from the horizontal deflection output circuit 1 and supplies the boosted output (output voltage) to the anode 16 of the cathode-ray tube.
Generally, as shown in FIGS. 1 and 3, the high voltage coil 12 is wound in multiple layers. If the coils of all layers are wound in the same count of turns, same winding width and same winding pitch and the end of the coil of each layer and the beginning of the coil of the next layer are connected with said diode 17 to have the same polarity, the potential difference is zero in terms of the alternating current between the coils of layers. Accordingly, the electric insulation between layers can be taken into consideration only as to the potential difference of the direct current and it is easy because heat generation due to dielectric loss need be taken into account.
If the high voltage coil is wound in multiple layers, the distance between the low voltage coil 11 and the high voltage coil 12 can be smaller than that of other section coils wound on the bobbin and the finish outside diameter of the high voltage coil can be small. Consequently, said high voltage coil 12 is advantageous in that its leakage inductance can be small. For this reason, the flyback transformer 2 employing the high voltage coil wound in multiple layers is widely used.
However, in case the high voltage coil 12 is only wound in multiple layers, the output current I.sub.H flowing from said coil 12 to the anode 16 of the cathode-ray tube 15 as shown in FIG. 4 suddenly reduces in the range of 0 to 200 .mu.A and this phenomenon is unfavorable. In recent years, a fixed resistor 18 and a variable resistor 20 are arranged in series connection between the high voltage output side (the anode side of the cathode-ray tube) and the grounding side to divide a current as much as approximately 10% of output current I.sub.H as shown in FIG. 1 and sudden reduction of said output current is prevented as shown in FIG. 5.
In other words, as shown in FIGS. 4 and 5, if the tolerance range of reduction of current I.sub.H is set to 0 to 1000.mu.A, th eoutput impedance Z.sub.01 =(27-25) kV/1000.mu.A=2M.OMEGA. as known from FIG 4. as far as the means for shunting current I.sub.H is not provided, whereas the output impedance Z.sub.02 is Z.sub.02 =(26.1-24.9)kv/1000.mu.A=1.2M.OMEGA. as known from FIG. 5 as far as the means for shunting current I.sub.H is provided, thus the output impedance has been substantially improved.
Recently, however, the demands have been more and more emphasized for improvement of the definition and precision of the cathode-ray tue 15 and further reduction of the output impedance. Moreover, the means which does not incur power loss in such reduction of the output impedance is strongly demanded but a method for shunting current I.sub.H through the fixed resistor 18 and the variable resistor 20 as described above has been unable to meet these demands and has not been accepted by the market.